greater understanding of language contact types. The works of Sarah Thomason and Terrence Kaufman have been used in particular to model borrowing and language shift. The research uses investigations into varieties of “Celtic” English (that is Welsh English, Irish English, etc.) which reveal characteristics more certainly attributable to Celtic languages and also universal contact trends revealed by other varieties of English. Old English Diglossia model Endorsed particularly by Hildegard Tristram (2004), the Old English diglossia model proposes that much of the native Romano-British population remained in the northern and western parts of England while the Anglo-Saxons gradually took over the rule of these regions. Over a long period, the Brittonic population imperfectly learnt the Anglo-Saxons' language while Old English continued in an artificially stable form as the written language of the elite and the only version of English preserved in writing. After the Norman conquerors removed Anglo-Saxon rule, the dialects of the general population, which would have included Brithonic and Norse-influenced versions of English, were eventually recorded and appear as Middle English. This kind of variance between written and spoken language is attested historically in other cultures, notably Latin, and may occur commonly. For instance, Moroccan Arabic (Darija) and other colloquial varieties of Arabic have had virtually no literary presence in over a millennium; the substantial Berber substratum in Darija (and likewise, the Coptic substratum in Egyptian, etc.) would not have appeared in any significant Arabic works until the late 20th century, when Darija, along with the other varieties of Arabic, began to be written down in quantity. The notion that such a diglossia could have existed in England, however, has been challenged by several linguists. Robert McColl Millar, for example, has pointed out that many works written in Old English, such as Ælfric's homilies, seem to be intended for a “large and undifferentiated audience,” suggesting that the language they were written in was not different from the language of the common people. He further concludes that “the idea that this state could continue for hundreds of years seems most unlikely,” noting further that no document from the time alludes to such a situation (by contrast, in Gaul, references are made to the lingua romana rustica as being different from written Latin).<ref>Robert McColl Millar, At the Forefront of Linguistic Change: The Morphology of Late Northumbrian Texts and the History of the English Language, with Particular Reference to the Lindisfarne Gospels</ref>